Looking for Duggan of Sydney Australia

I am looking for information on John and Mary Duggan who lived in the eastern suburbs of Sydney around 1900. As far as I know they came to Australia from Cork, Ireland and had 5 children one of whom was my grandfather, Frederick David Duggan who was born 1903? and married my grandmother Irene Bourke in 1928. Frederick died in 1950 from injuries sustained in WWII. I have no information about his sibblings or anthing else on his parents.
I am writing a family history for future generations and would love to trace the line back to Ireland.